Move xToSqlSelect to sql.Table
Move expressionToSql to relational.Expr.toSqlExpr
Refactor table -> query -> select into method
Add UNION ALL
Refactor stringifying CombineOp
Have Const take Any as value, not String
Add method to create sql literal value from Any type
Document Domain
Return optional literal instead of throwing exception in sqlLiteralValueOf
Turn some FIXMEs to TODOs
Document toSqlExpr
Document DbConnection
Document Query